This invention relates to a vertical free surface type pump and more particularly, to a liquid metal mechanical pump equipped with an emergency syphon system for preventing the liquid metal free surface inside the pump from rising to an upper mechanical bearing of the pump at the time of failure of a cover gas line.
A vertical, free surface type mechanical pump having a construction which has the free surface of the liquid metal inside a pump casing and in which a cover gas space is disposed over the free surface of the liquid metal is the type of pump most usually used as a primary circulating pump for a loop type liquid metal cooled fast breeder reactor. An example of such a mechanical pump is illustrated in FIG. 1A. Liquid sodium flows into a sustantially cylindrical casing 1 from a suction nozzle 2 at the lower end of the casing 1, is given a delivery pressure by an impeller 3 and flows out through a delivery nozzle 4. A pump drive shaft 7 for transmitting the rotating force to the impeller 3 is pivotally supported by a lower hydrostatic bearing 8 and an upper mechanical bearing 9 defining an overflow space between them. The liquid metal that enters the overflow chamber overflows to an overflow column 6 through an overflow pipe 5 is returned to the suction nozzle 2. A mechanical seal 10 for preventing the leakage of a cover gas (i.e. an inert gas) from the casing 1 is disposed below the mechanical bearing 9. The inert gas is caused to constantly flow downwards from the mechanical seal 10 in order to prevent the vapor of the liquid metal from rising into the seal and, at the same time, to apply a predetermined cover gas pressure, thereby setting the level of the free surface inside the pump and providing a required suction head necessary for the pump.
In the example of the prior art pump shown in FIG. 1A, the cover gas is supplied from a gas feed pipe 11 connected to the lower portion of the mechanical seal 10, descends through the gap between a shield 12 and the shaft 7, then enters a cover gas space 13 and is recovered through a gas discharge pipe 14 connected to the overflow column 6 and through an exhaust pipe 15. Thus, the cover gas circulation is effected. This cover gas pressure is higher than the atmospheric pressure of the atmosphere in which the pump is disposed. Accordingly, if the cover gas line or piping for the above-described cover gas circulation is accidentally broken, the free surface inside the pump drastically rises and, at times, it reaches the mechanical seal 10 as well as the mechanical bearing 9, thus causing serious damage to the pump and inviting leakage of the liquid metal.
The free surface inside the pump and the free surface inside the overflow column 6 shown in FIG. 1A are considered to be those when the pump is in the normal operation. FIG. 1B shows the changes in the free surfaces inside the pump and inside the overflow column when the pump is used as a primary circulating pump of a primary cooling system of a liquid metal cooled fast breeder reactor and the gas feed pipe 11 or the exhaust pipe 15 is broken. The ordinate in FIG. 1B corresponds to the height in FIG. 1A and the abscissa represents the elapsed time after the gas line failure. The curves in FIG. 1B have the following meanings. Reference numeral 20 designates the curve representing the pump free surface; reference numeral 21 designates the curve representing the overflow column free surface; reference numeral 22 designates a curve representing a cover gas pressure in a reactor vessel (represented by the head of the liquid metal); reference numeral 23 designates the upper end position of the gas line 14; reference numeral 24 designates the lower surface position of the shield plug 12; and reference numeral 25 designates the position of the gas feed pipe 11.
In this case, since the cover gas line of the pump is communicated with a cover gas system of a reactor vessel 30 as shown in FIG. 2, the gas pressure inside the pump drops down to the atmospheric pressure within a short period of time if the gas line is broken in the proximity of the pump. On the other hand, it is known that since the capacity of the gas space 31 in the reactor vessel 30 is far greater than that of the pump, it takes more than one minute before the cover gas pressure on the side of the reactor vessel drops down to the atmospheric pressure. In the interim, unbalance of the gas pressure develops between the reactor vessel and the pump and this pressure difference raises the free surface inside the pump. In other words, the free surface inside the overflow column 6 rises simultaneously with the failure of the gas line and, when it reaches the same level as the free surface inside the pump, both rise together. However, when the surface reaches the upper side of the gas line 14, the free surface inside the overflow column 6 can not rise very much any longer and only the free surface inside the pump continues rising. At this point, the cover gas pressure in the reactor vessel 30 still has a high pressure so that the free surface inside the pump reaches the shield plug 12, the mechanical seal 10 and the mechanical bearing 9, thereby not only causing serious damage to the pump but also inviting leakage of the liquid metal outside the pump. This danger becomes greater in the case of the cold leg pump arrangement because a cover gas pressure during operation is higher than that of a hot leg pump arrangement.
In FIG. 2, reference numeral 32 designates an outlet nozzle; reference numeral 33 designates a main cooling piping; reference numeral 34 designates a drain valve; reference numeral 35 designates a drain tank; and reference numeral 36 designates a cover gas refiner.